The present invention relates to an overdischarge prevention circuit for a rechargeable battery.
A rechargeable battery that is repeatedly usable by recharging suffers a reduced battery quality and a shortened battery life when the rechargeable battery is allowed to discharge such that the battery voltage of the same becomes lower than a predetermined value. In order to prevent such overdischarging of a rechargeable battery, a circuit has been adopted in which the battery voltage of the rechargeable battery is detected and discharging of the rechargeable battery is interrupted when the detected battery voltage becomes lower than a predetermined value. For example, in Japanese Non-examined Patent Publication No.4-33271 published Feb. 4, 1992, an overdischarge prevention circuit is described in which a switch element is connected in series to the rechargeable battery, the switch element being so constructed as to be turned OFF when the battery voltage of the rechargeable battery becomes lower than a predetermined value. Whereby discharging of the rechargeable battery is interrupted.
In the overdischarge prevention circuit described in the noted Japanese Patent Publication, the switch element connected in series to the rechargeable battery is kept in an OFF state until the battery voltage of the rechargeable battery becomes somewhat higher than the predetermined value, and when the battery voltage becomes somewhat higher than the predetermined value, the switch element is turned ON to start discharging again.
In the above-mentioned overdischarge prevention circuit, a hysteresis is provided between a voltage for interrupting discharging and a voltage for starting discharging again. This hysteresis is provided for preventing chattering, namely, repeated turning ON and OFF of the switch element caused by the rise of the battery voltage of the rechargeable battery through self resetting after the interruption of discharging. An overdischarge prevention circuit provided with a hysteresis can prevent intermittent continuance of discharging from a rechargeable battery. However, in such a overdischarge prevention circuit, it is very difficult to determine the range of a hysteresis voltage between a voltage for interrupting discharging and a voltage for starting discharging again.
When the range of the hysteresis voltage is too large, discharging from the rechargeable battery cannot be started again until the rechargeable battery is sufficiently charged. On the contrary, when the range of the hysteresis voltage is too small,chattering of the switch element cannot be prevented.